Pulmonary surfactant lines the lung epithelium and lowers surface tension to prevent collapse at end-expiration. It is synthesized in epithelial type II cells and stored in lamellar bodies. Secretion of surfactant occurs upon fusion of lamellar bodies with plasma membranes. Inadequate amounts of pulmonary surfactant results in respiratory distress syndrome in premature infants. The factors that participate in and regulate the secretion of lung surfactant from alveolar type II cells is poorly understood. The long term goal of this research project is to elucidate the molecular mechanism responsible for exocytosis of lamellar bodies, especially, the role of annexins in lung surfactant secretion. Annexins are a family of Ca2+-dependent phospholipid-binding proteins and have been implicated in exocytosis. N-terminal modifications of annexins in vitro reduce Ca2+ requirement for annexin-mediated membrane fusion to physiologically attainable levels and could be a physiologically regulated process. The present study therefore proposes to evaluate the roles of annexins I and II, and their posttranslational modifications in lung surfactant secretion. The hypothesis will be tested by: (1) in vivo study using isolated intact type II cells; (2) in vitro study of fusion of isolated lamellar bodies with plasma membranes and (3) using permeabilized type II cells. In specific, posttranslational modifications of annexins (proteolysis and phosphorylation) will be examined in type II cells stimulated with secretagogues (terbutaline, ATP and ionophore A23187). Such secretagogue-induced modification will be correlated to the augmented secretory activity in the isolated type II cells. Second, purified annexins will be modified in vitro by proteolysis, phosphorylation and deglycosylation and the fusion of lamellar bodies with plasma membranes mediated by the native or modified annexins will be compared. Third, essential role of annexins will be established by the reconstitution of secretion from permeabilized type II cells with exogenous annexins. The information obtained from this study may provide a deeper insight into the key determinants of pulmonary function and possible dysfunction and give a value direction to the therapy of pulmonary diseases such as neonatal respiratory distress syndrome.